The present invention relates to a method for quasi halftone processing of image that is used in printers, scanners, copy-machines and facsimile, and in which lower-gradation image data is reproduced by the modulation of a high or multi-gradation image into an image having a smaller number of gradations.
There is a systematic dither approach (hereinafter, referred to as dither) as one of the methods for quasi half-tone processing of multi-gradation images. We shall describe a binarizer for converting a multi-gradation image to a binary image by this conventional dither.
FIG. 7 is a diagram showing the construction of the conventional binarizer using the dither, and FIG. 8 shows the conventional dither matrix.
The binarizer will be briefly described with reference to FIG. 7. Referring to the figure, image data 1 is original multi-valued data to be binarized. These data to be binarized for use in printers are usually image data having four color components of black, cyan, magenta and yellow. Dither matrix storage unit 3 has a dither matrix stored therein as a table of threshold values shown in FIG. 8. This table is an example of the dither matrix that is used when the thickness of image data 1 has a range of 256 gradations from level 0 to level 255. This matrix data has so far been designed so that dots can be regularly arranged on the basis of a certain generation rule. There is also shown a comparator 2, which receives thickness data N of each pixel of each color component of the image data 1, and threshold data T corresponding to the coordinates of this image data from the dither matrix storage means 3. The comparator 2 then compares the image data N and the threshold data T to produce a binarization result Q of 1, or a binary signal of dot ON when N>T, and the binarization result Q of 0, or a binary signal of dot OFF when N<T. These processing operations are performed for all the pixel data of each of the color components that constitute the image data, thus finally generating binary image data.